Power supplies operating directly off of an AC line typically comprise an input rectifier to convert the input AC line voltage to a DC voltage. This DC voltage is typically applied to a charge storage capacitor which is connected to the input of a DC/DC converter, which conveys this DC voltage to a DC voltage of another level at its output. The operating characteristics of the rectifier and storage capacitor inherently distort the input current waveform at the input to the rectifier. This waveform distortion causes harmonics to be fed back onto the AC line and further leads to significant EMI emissions and to unnecessary power losses in the AC distribution circuitry.
In other applications power supplies may operate directly off of an AC line to drive a frequency changer to provide a signal at a different frequency from that on the AC line. The input waveform may be distorted leading to the difficulties described above.
The distortion of the input current waveform may be controlled by using an active power factor controller and a converter, such as a boost converter, inserted between the rectifier and the storage capacitor to actively control the input current waveform. The active switching device is controlled in response to a control circuit that monitors the input voltage waveform. The control modulates the conduction intervals at a high frequency compared with the AC line frequency, so that the wave form of the input current is constrained to conform to that of the input voltage waveform or to the fundamental sinusoidal waveform of that input voltage waveform.
Active power factor control networks typically sense input and output signal parameters of the power circuit and utilize a power switch selectively switched or pulse width modulated in response to these signal parameters to force the input current to conform to a desired current waveform. In a particular illustrative arrangement disclosed in U.S. Pat. No. 4,412,277 a rectified input voltage waveform is multiplied with an error voltage representing the deviation of the output voltage from a regulated value. The resulting control signal is properly scaled and used to control the modulating pulse driving the power switch to provide the desired input current waveform. In a more sophisticated power factor control arrangement disclosed in U.S. Pat. No. 4,677,366 a feedforward control is added to compensate for rapid change in the rms input AC voltage. This is used to inversely scale the programmed input current by the square of the rms input voltage.
A limitation with these existing arrangements is the effect of ripple existing in the sensed voltage waveforms which have undesirable effects in the operation of the control circuitry resulting in an inaccurate determination of the waveform of the programmed input current. At present techniques to deal with this ripple current lengthen the response time of the power factor correction circuitry resulting in substantial transient signals in the output voltage unless a large output charge storage capacitor is used.
The success of the controller in generating the correct current waveform is dependent on the speed and accuracy with which it derives or estimates the V.sub.rms.sup.2 or equivalently the V.sub.peak.sup.2 of the input voltage for use by the controller in controlling the boost converter's active power switch. This quantity has traditionally been derived by full-wave rectifying the input AC sine wave, filtering the rectified sine wave and squaring the filtered signal. While this technique has the advantage of simplicity of circuitry and of its inherent operation, it has the disadvantage of requiring a design trade off between ripple in the derived squared voltage and the speed with which the circuit can respond to dynamic changes in the input voltage.